For example, a conventional ring rolling mill disclosed in Patent Document 1, etc. rolls a peripheral portion of a ring-shaped body in a radial direction while the ring-shaped body is rotated in its peripheral direction, with the peripheral portion pinched in the radial direction between an outer peripheral surface of a main roll which is rotationally driven, and an outer peripheral surface of a mandrel which is rotatable. Also, in the conventional ring rolling mill, the peripheral portion of the ring-shaped body is rolled in the radial direction by the outer peripheral surfaces of the main roll and the mandrel by relatively bringing or separating the main roll and the mandrel close to or from each other in a state where their axes of rotation are kept substantially parallel to each other.
However, in the conventional ring rolling mill, the main roll and the mandrel are brought close to or separated from each other in a state where their axes of rotation are kept substantially parallel to each other. Thus, the pressing forces applied on the peripheral portion of the ring-shaped body by the main roll and the mandrel could be made different in every peripheral position on the peripheral portion, but could not be made different in every position in the thickness direction. That is, the pressing forces could not be made different locally in the peripheral portion of the ring-shaped body.    Patent Document 1: Japanese Patent Publication No. 2859446